ANNUAL RED CRAB MIGRATION: EFFORTS TO PROTECT A KEY FOREST SPECIES

By Gundhramns Hammer

June1, 2013

Source: Australian Government.

Every year at the beginning of the wet season (October/November), more than 50 million adult red crabs (Gecarcoidea natalis) set out on an epic journey from the forest to the coast to breed. 

This phenomenon happens on Christmas Island, a small Australian island in the Indian Ocean. The red crabs are endemic to this island. The annual red crab migration is so spectacular that has become a tourist attraction.

Males are the first to start the migration to the beach and later females will follow them (Fig. 1). The joined forces of the migrating red crabs form a giant red moving wave (Fig. 2).


 

Figure 1. Diagram of male and female Christmas Island Red Crab (Gecarcoidea natalis). Source: Australian Government.

Figure 2. A wave of Red Crabs (Gecarcoidea natalis) crossing a road. Source: Australian Government.

It takes the crabs 5-7 days to reach the sea (Fig. 3) where they breed for over a month. Following a lunar phase, females spawn 17 to 18 days after mating (Adamczewsk & Morris, 2001). 

Figure 3. Red Crabs (Gecarcoidea natalis) on the beach. Source: Australian Government.

The females retreat to fresh burrows dug by males and stay there for 12-13 days to develop up to 100.000 eggs. Females release the eggs into the sea on a high pre-dawn  tide in December and January.

The eggs hatch immediately, giving rise to 4 mm larvae which will lead a planktonic life for about a month. When the larvae reach the stage called megalops with a diametre of approximately 4mm, they come back and gather on the shoreline. 

The megalops undergo a metamorphosis into air breathing tiny crabs in the water on the shoreline (Fig. 4). After a month of growth, the small crabs (6-8 mm in diametre) leave the water and head to the forest along with the adults which are on their way back home.

Figure 4. Life cycle of the Christmas Island Red Crab (Gecarcoidea natalis). Source: Australian Government.

Young crabs spend most of their time underground in burrows in the forest. If it rains they may come out to forage for leaves and other debris they find on the forest floor.

Red crabs (Gecarcoidea natalis) are diurnal and very sensitive to moisture in the air. During the dry season, the crabs retreat into their moist burrows. They close the burrow entrance with a plug made up of loose leaves and this keeps the burrows with a high humidity.

Careful drivers

The days when thousands of red crabs used to be crushed by cars on the roads are over. Nowadays people have realised the importance of these crustaceans and do their best to protect and avoid killing crabs on the roads. 

Furthermore, the Australian Government has a crew of park staff that works hard preparing for the spectacular annual migrations of the red crabs.

What do the park staff do? They set up temporary fencing to keep the crabs off the roads (Fig. 5). The crabs do the rest. The animals crawl along the fencing until they reach special crab crossings (Fig. 6). The crossings are cleaned once a year of any accumulated debris.

Figure 5. temporary fencing to keep red crabs off the roads. Source: Australian Government.

 

Figure 6. Crab crossing. Source: Australian Government.

Some roads are temporarily closed (Fig. 7) to help the migrating red crabs. At other points, special overpass bridges are constructed for the crabs (Fig. 8).

Figure 7. Road closed sign to help the migrating crabs. Source: Australian Government.

Figure 8. Overpass bridge for migrating crabs. Source: ABC Goldfields.

Local people also cooperate in the protection of the migrating crabs. They join the park staff with garden rakes to keep the crabs from going under the cars. The result is that more crabs reach the coast to spawn (Video 1).

           Video 1. Park staff preparing the roads for the annual Red Crabs (Gecarcoidea natalis) migration.

Red crabs, a key species in the forest

Red crabs (Gecarcoidea natalis) play an important role in the forest ecosystem. Their diet consists of fallen leaves, tree branches, flowers, fruits, seedlings and carcasses of crabs, birds, mollusks, and other dead animals. And if they live in a garden of the locals, the crabs will also eat food scraps should there be a chance.

They act as a cleaning crew and their droppings act as a fertilizer on the forest floor.Their burrowing turns over and aerates the soil. Their selective browsing on seeds and seedlings determines the structure and composition of the Christmas Island forest (Green, 1993).


Man acting as a good warden

The interaction of red crabs and humans on Christmas Island is positive for both sides. The crabs are protected and the local people get benefits from ecotourism.

This is a good example of man doing his best to live in harmony with nature. 



References

Adamczewsk A. M. & Morris S. (2001). Ecology and Behavior of Gecarcoidea natalis, the Christmas Island Red Crab, during the Annual Breeding Migration. Biol. Bull., 200: 305-320.

Green P. T. (1993). The Role of the Red Land Crabs (Gecarcoidea natalis (Pocock, 1888); Brachyura, Gecarcinidae) in Structuring Rain Forest on Christmas Island, Indian Ocean. Ph.D. Thesis, Monash University, Clayton, Victoria, Australia. 304 p.